Among the illnesses linked to obesity are breast and colorectal cancers. At a science writers' seminiar sponsored by the American Cancer Society this week in San Diego, researchers described studies aimed at determining the role of fat in cancer, and whether reducing fat can have a protective effect.

About 40 percent of the calories in the average U.S. citizen's diet come from fat. Based on epidemiological data and animal studies, anticancer diets (SN: 10/1/83, p. 217) recommend a level of no more than 30 percent. But the effect of lowering fat intake awaits proof.

So the National Cancer Institute (NCI) in Bethesda, Md., has just kicked off a 10-year, $30 million trial. According to NCI's Peter Greenwald, 12,000 women at high risk of breast cancer will be enrolled.

Half the women will be encouraged to reduce their fat intake to 20 percent of total calories by such dietary changes as switching from whole milk to skim milk products, trimming fat from meat and avoiding fried foods. Because the reduction is a dramatic one and might not be easy to achieve, Greenwald notes, the early part of the study will be devoted to determining its feasibility.

A second study, also just begun, will involve 2,000 women who have had mastectomies; researchers want to see if the 20 percent fat diets will reduce the risk of recurrence.

The role of obesity in breast and colorectal cancer shows up clearly in animal studies by David Kritchevsky of the Wistar Institute in Philadelphia. In one experiment he allowed 24 female rats to eat as much as they wanted; 14 of those developed breast cancer after being given a carcinogenic chemical. But none of the 24 rats that received the same chemical and were fed only 60 percent of the calories consumed by the first group developed mammary tumors.

In a colon cancer experiment, 19 male rats eating as much as they wanted got chemically induced cancer, while only 53 percent--10 of 19--eating 60 percent of the calories got cancer.

Is it the reduction of fat, or of calories? Kritchevsky says both may play a role. "Maybe this will give a little more latitude in preparing a diet," he says.

Just how fat intake or obesity aids in promoting cancer isn't known. greenwald suspects that the way fat alters that level of certain hormones may be a factor in some aspects of the development of cancer, while Kritchevsky suggests that low caloric intake may starve out tumors.